A reconfigurable optical add and drop multiplexer (ROADM) has been known, for example, as a transmission device that wavelength-multiplexes a plurality of optical signals having different wavelengths for a transmission. In setting a path within a network including transmission devices each wavelength-multiplexing and transmitting a plurality of optical signals having different wavelengths, the transmission quality of an optical signal is estimated by calculating cumulative values of attenuation, noise, waveform distortion and others of the optical signal that occur in a transmission device or transmission line on a path of the optical signal, based on a transmission characteristic of the transmission device or transmission line. Then, the estimated transmission quality and a required transmission quality are compared with each other, and when the path is determined to be able to normally transmit the optical signal as a result of the comparison, the path is set in the network.
In order to estimate the transmission performance of an optical signal path and determine whether the path is able to perform a transmission, a network management device is equipped with a transmission characteristic database in which various data related to the transmission characteristic of an optical signal in a transmission line or transmission device are registered. Since the transmission characteristic of an optical signal varies depending on combinations of wavelengths or powers of optical signals to be wavelength-multiplexed with the optical signal, the data registered in the transmission characteristic database are only related to the transmission characteristic of a predetermined combination that is estimated in designing the network.
With regard to the transmission characteristic, for example, Japanese Laid-Open Patent Publication No. 2002-057624 discloses that, in order to suppress the difference in the transmission characteristics of the optical signals of the respective wavelengths, the setting of a transmitting side of a wavelength-multiplexed optical signal is feedback-controlled according to a bit error rate or the like measured by a receiving side of the optical signal.